Oligosaccharides from human milk samples obtained from individual donors were analyzed using high-pH anion-exchange chromatography. Three patterns of neutral oligosaccharides were detected corresponding to milk groups already described. These oligosaccharide groups correspond to the Lewis blood types Le(a-b+), Le(a+b-), and Le(a-b-). A new carbohydrate pattern was detected in a milk sample from a Le(a-b-) person in which only nonfucosylated oligosaccharides and compounds bearing alpha1,3-linked fucosyl residues were found. This finding led to the hypothesis that there exist 4 different oligosaccharide milk groups that fit well to the genetic basis of the Lewis blood group system.